1) Field of the Invention
The present invention relates to a method for producing a secondary cell having a flat wound electrode body that inhibits the bending of the electrode board caused by charging and discharging, and resulting swelling of the cell and deterioration of cycle characteristics.
2) Description of the Related Art
In recent years, there has been a rapid reduction in the size and weight of mobile information terminals such as mobile phones, notebook personal computers, and PDAs. Secondary cells and batteries, which are the driving power sources of such terminals, are required to be easily mounted inside the mobile information terminals, and easy to exert a large amount of current from.
Non-aqueous electrolyte secondary cells such as lithium-ion secondary cells, which have square outer casings or laminate outer casings in which flat wound electrode bodies are housed, are easily mounted inside mobile information terminals and easy to exert a large amount of current from because of a large area of facing between the positive and negative electrodes. Thus, such non-aqueous electrolyte secondary cells are useful as the driving power sources of mobile information terminals.
In such non-aqueous electrolyte secondary cells, the positive and negative electrode boards repeat swelling and contraction because of intercalation and deintercalation of lithium ions through charging and discharging. The adhesive (adhesive tape) that fixes the winding end of the flat wound electrode body restricts the swelling and contraction of the electrode boards. As shown in FIG. 8, this causes the swelling of the electrode boards toward the inside of the flat wound electrode body, which in turn causes bending of the electrode boards. The bending of the electrode boards has presented the problem of increasing the thickness of the cell. In addition, at the bent portion, a gap occurs between the positive electrode board and negative electrode board. At this gap the smooth movement of lithium ions are interrupted, presenting the problem of increased deterioration of cycle characteristics.
In order to solve these problems, the following attempt is being made. By using a winding core having a flat (oval, polygonal, and the like) cross section, and by reducing the tension applied on the electrode boards at the time of winding, a space is provided between the electrode boards of the flat wound electrode body so that the space adsorbs the swelling of the electrode boards, thus attempting to inhibit the bending of the electrode boards. However, since the cross section of the winding core is flat, there is a problem that the speed of winding needs to be reduced compared with the case of a circular cross section for the winding core. In addition, since the tension applied on the electrode boards is small, there is a problem of a reduction in the quality of winding (the electrode boards and separator are easily displaced when wound).
Techniques related to the non-aqueous electrolyte secondary cell are proposed in patent documents 1 to 4, and the outlines are as follows.
Patent Document 1: Japanese Patent Application Publication No. 2003-157888 (claims and paragraphs 0005-0009).
Patent Document 2: Japanese Patent Application Publication No. 2003-338307 (claims and paragraphs 0013-0029).
Patent Document 3: Japanese Patent Application Publication No. 11-121044 (paragraphs 0004 and 0005).
Patent Document 4: Japanese Patent Application Publication No. 11-176476 (paragraphs 0004-0009).
(i) Patent documents 1 and 2 propose the following technique. In order to absorb the deforming stress of the electrode boards, a space portion is provided between two neighboring strip laminates of a group of flat wound electrode boards (flat wound electrode body). The two neighboring strip laminates are on the long axis line of a cross section perpendicular to the winding axis of the group of flat wound electrode boards.
According to this technique, since the space portion absorbs the deforming stress of the electrode boards, bending caused by swelling and contraction of the electrode boards is eliminated. However, with this technique, a spacer is used when preparing a cylindrical electrode body, and therefore, after pressing the body flat, the step of removing the spacer is required. It is difficult, however, to wind the electrode boards with the spacer on, presenting the problem of poor productivity.
(ii) Patent document 3 proposes the technique of using, as the adhesive for fixing the winding end of the cell element (wound electrode body), a material that swells, dissolves, or decomposes after contact with the electrolytic solution.
According to this technique, since the swelling, dissolution, or decomposition of the adhesive provides relaxation to the winding of the cell element, the stress applied on the constituents of the element is alleviated, thereby preventing associated deterioration of cell characteristics.
However, with this technique, the relaxed winding of the cell element causes a poor state of facing between the positive and negative electrode boards in the cell element. This interrupts the smooth progress of the charge-discharge reaction, presenting the problem of deteriorating cell characteristics such as cycle characteristics.
(iii) Patent document 4 proposes the technique of using, as the adhesive for fixing the cell element (wound electrode body), a material that, while substantially not dissolving or decomposing in the electrolytic solution, lowers its function of fixing the cell element after contact with the electrolytic solution.
According to this technique, since the function of fixing the cell element is lowered after the adhesive comes in contact with the electrolytic solution, and the winding of the cell element is thereby relaxed, the stress applied on the constituents of the element is alleviated, thereby preventing associated deterioration of cell characteristics. Also, this prevents contamination of the electrolytic solution which is caused by dissolution of the adhesive in the electrolytic solution. However, with this technique, similarly to the technique of patent document 3, there is a problem that cell characteristics such as cycle characteristics deteriorates.